1. Field of the Invention:
The invention in general relates to steam turbines having bypass arrangements, and more particularly to a system for speed control of the turbine unit.
2. Description of the Prior Art:
In the operation of a steam turbine power plant, a boiler produces steam which is provided to a high pressure turbine section through a plurality of steam admission valves. Steam exiting the high pressure turbine section is reheated, in a conventional reheater, prior to being supplied, through another valving arrangement, to an intermediate pressure turbine section (if included) and thereafter to a low pressure turbine section, the exhaust from which is conducted into a condenser where the exhaust steam is converted to water and supplied to the boiler to complete the cycle.
The regulation of the steam through the high pressure turbine section is governed by the positioning of the steam admission valves and as the steam expands through the turbine sections, work is extracted and utilized by an electrical generator for producing electricity.
A conventional fossil fueled steam generator, or boiler, cannot be shut down instantaneously. If, while the turbine is operating, a load rejection occurs necessitating a turbine trip (shutdown), steam would normally still be produced by the boiler to an extent where the pressure increase would cause operation of various safety valves. In view of the fact that the steam in the system is processed to maintain a steam purity in the range of parts per billion, the discharging of the process steam can represent a significant economic waste.
Another economic consideration in the operation of a steam turbine system is fuel costs. Due to high fuel costs, some turbine systems are purposely shut down during periods of low electrical demands (for example, overnight) and a problem is encountered upon a hot restart (the following morning) in that the turbine has remained at a relatively hot temperature whereas the steam supplied upon boiler start-up is at a relatively cooler temperature. If this relatively cool steam is admitted to the turbine, the turbine would experience thermal shock which would significantly shorten its useful life. To obviate this thermal shock the steam must be admitted to the turbine very slowly, thereby forcing the turbine to cool down to the steam temperature, after which load may be picked up gradually. This process is not only lengthy, it is also costly.
As a solution to the load rejection and hot restart problems, bypass systems are provided in order to enhance process on-line availability, obtain quick restarts, and minimize turbine thermal cycle expenditures. Very basically, in a bypass operation, the steam admission valves to the turbine may be closed while still allowing steam to be produced by the boiler. A high pressure bypass valve may be opened to divert the steam (or a portion thereof) around the high pressure turbine section, and provide it to the input of the reheater. A low pressure bypass valve allows steam exiting from the reheater to be diverted around the intermediate and low pressure turbine sections and be provided directly to the condenser.
Normally the turbine extracts heat from the steam and converts it to mechanical energy, whereas during a bypass operation, the turbine does not extract the heat from the bypassed steam. Since the elevated temperature of the steam would damage the reheater and condenser, relatively cold water is injected into the high and low pressure bypass steam paths so as to prevent overheating of the reheater and condenser.
Normally in a nonbypass operation, the turbine is accelerated from turning gear to some predetermined speed by slowly opening the steam admission valve to the high pressure turbine, with the valving arrangement to the intermediate pressure turbine being wide open. This type of operation is not suitable with a bypass arrangement since a relatively high pressure is maintained at the output of the reheater and would result in an uncontrolled steam flow through the intermediate pressure turbine tending to the unbalance the high pressure to intermediate pressure load ratio and subjecting the turbine to undesired mechanical and thermal stresses.